There is a general desire to improve the capacity of switches/routers in optical telecommunication systems. One approach is to employ wavelength division multiplexed signals and wavelength selective switches (WSSs), which may be employed in a reconfigurable optical add/drop multiplexer (ROADM) or the like. Another approach to employ data encoding based upon polarisation, and U.S. Pat. No. 7,397,980 describes a switching device for such signals. Further background prior art can be found in U.S. Pat. No. 7,397,980; in U.S. Pat. No. 7,664,395; and in M. Okuno and H. Takahashi, “Recent Progress in Optical Switches Based on Planar Lightwave Circuit Technology”, NTT paper at IEEE Conference on All-Optical Networking, WD2-55 (2002). However there is a general need for improvement on the prior art approaches.